Commonly, hydrocarbon streams produced by refining operations or natural gas liquids processing operations contain phenolics and sulfur compounds, such as hydrogen sulfide and mercaptans. These streams are usually treated with alkaline solutions such as aqueous solutions of an alkali metal hydroxide in order to remove such sulfur compounds and phenolics and reduce odor and corrosivity associated with these acidic species. For example, mercaptans are generally removed from such hydrocarbon streams by washing or contacting such streams with an aqueous solution of sodium hydroxide or potassium hydroxide. The sulfur compounds are absorbed into the alkaline solution and reacted with it to form sulfides and mercaptides, i.e., the metal salts of hydrogen sulfide and the mercaptans respectively. Also any phenolics contained in thermal or catalytic cracked hydrocarbons are typically converted to phenolates when absorbed into the alkaline solution. The alkaline solution containing the sulfides, mercaptides, and phenolates is then typically separated from the hydrocarbon stream, regenerated and recycled for contacting more of the sulfur-containing hydrocarbon stream, until its alkalinity is reduced below effective levels. Effluent alkaline solutions that contain toxic contaminants such as hydrogen sulfide, or have a pH value greater than 12.5 are defined by the U.S. Environmental Protection Office as characteristic hazardous waste. Consequently, they require special handling prior to disposal.
Existing processes, examples of which are discussed herein, deal primarily with managing the fresh caustic requirements of a refinery and thus focus in regenerating the effluent caustic by removing most of the sulfides and mercaptides. For instance, one method of regenerating the alkaline solution is air/steam stripping of the mercaptides from the alkaline solution. In yet another method, the sulfides and mercaptides are oxidized in the presence of oxygen in an oxidation zone to thiosulfate and disulfides. The thiosulfates remain dissolved in the alkaline solution and reduce the alkalinity of the alkaline hydroxide solution. The disulfides, which are relatively insoluble in the alkaline solution, may then be removed therefrom by contacting the alkaline solution with an organic solvent. In order to carry out the oxidation, a catalyst is generally employed. The catalyst includes certain metal chelates, for example, phthalocyanine disulfonate. The oxidation zone typically involves a column having suitable contacting means such as trays with bubble caps, suitable spacing material such as Raschig rings or a plurality of fibers positioned longitudinally in the column.
U.S. Pat. No. 2,921,021 to Urban et al. relates to the treatment of a sour hydrocarbon distillate with an alkaline solution. The effluent alkaline solution containing mercaptides is then mixed with air in a regenerator whereby the mercaptides are oxidized to disulfides. The regenerated caustic and disulfides are in the form of a finely dispersed mixture. The dispersion is passed through a coalescing system and then to a settling tank whereby the disulfide compounds are separated from the alkaline solution. While most of the disulfides are removed in the settling tank, in some cases the settling step may be followed by a naphtha wash to remove disulfides still retained in the alkaline solution.
U.S. Pat. No. 2,853,432 to Gleim et al. discloses the regeneration of used alkaline reagents by oxidizing same using a phthalocyanine catalyst. For example, mercaptides contained in a caustic solution were oxidized to disulfides, which were then withdrawn from the regeneration zone by skimming or by dissolving in a suitable solvent such as naphtha.
U.S. Pat. No. 3,574,093 to Strong relates to a multi-step process wherein the effluent caustic solution generated by treating a low-boiling hydrocarbon stream for mercaptan removal is thereafter used in a second treating step wherein a higher boiling sour distillate is sweetened. In the sweetening step, the mercaptans in the sour distillate are oxidized to disulfides. The disulfides exit the treating stage in the hydrocarbon stream along with those mercaptides which had been previously extracted and oxidized from the low boiling hydrocarbon stream. Thus, the higher boiling stream is sweetened at the same time. The regenerated caustic solution is then introduced into a separation zone from which the disulfide phase is recovered from the caustic zone. The coalescence of the disulfide compound into a separate phase is stated to be extremely difficult without the use of coalescing agents. In addition, a high residence time is used in the separation zone to further facilitate this phase separation.
U.S. Pat. No. 4,362,614 to Asdigian also relates to a multi-step process for the extraction of mercaptans from hydrocarbon streams with an alkaline solution, followed by the regeneration of the mercaptide-containing alkaline solution resulting from such extraction by oxidation in the presence of a catalyst in an oxidation zone, followed by the separation of the disulfides and the alkaline solution by decantation within a phase separation zone. From this process, the alkaline solution is recycled. In addition to the requirement of a separate oxidation zone and a large settling zone, the use of additional coalescing means is said to be required.
U.S. Pat. No. 4,666,689 to Maple et al. discloses regenerating a effluent alkaline solution by adding a suitable oxidation catalyst to the effluent alkaline solution and contacting the alkaline solution with an oxygen-containing solvent in a reaction zone, wherein the solvent is immiscible with the effluent alkaline solution. The reaction zone comprises a plurality of fibers positioned longitudinally within a conduit, whereby the two liquids, i.e., the effluent alkaline solution containing the oxidation catalyst and the oxygen-containing solvent, are in contact while co-currently flowing through the reaction zone during which the mercaptides are oxidized to disulfides and are simultaneously extracted from the alkaline solution to the solvent.
However, the aforementioned processes do not provide a satisfactory solution to the problem of disposal of the alkaline solution when after it has been used a number of times it is no longer suitable for any use in the refinery. Moreover, during a typical treatment of thermal or catalytic cracked hydrocarbon streams any phenolics contained therein are converted to phenolates, for instance cresylic acid is converted into sodium cresylate, which are subsequently incorporated into the alkaline stream. This has the undesired effect of reducing the volume of the hydrocarbon stream. Moreover, it requires special handling of the effluent alkaline solution for disposal purposes. Methods and associated equipment available to recover the cresylates are complex and very costly. Because the volume of the effluent alkaline solution generated by individual operators is relatively small, it is generally not economically feasible for these operators to regenerate such phenolate containing alkaline solutions. Some companies recover cresylates from effluent alkaline solutions which are transported to them, but, the additional transportation costs renders this option economically deficient or often times unfeasible.
Therefore, there is a need for a simple and inexpensive method for treating effluent alkaline solutions containing phenolates, sulfides and mercaptides to separate the aqueous alkaline solution for disposal or reuse.